.\" t
.TH STAPPROBES 3stap 
.SH NAME
stapprobes \- systemtap probe points

.\" macros
.de SAMPLE

.nr oldin \\n(.i
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.SH DESCRIPTION
The following sections enumerate the variety of probe points supported
by the systemtap translator, and some of the additional aliases defined by
standard tapset scripts.  Many are individually documented in the
.IR 3stap
manual section, with the
.IR probe::
prefix.

.SH SYNTAX

.PP
.SAMPLE
.BR probe " PROBEPOINT [" , " PROBEPOINT] " { " [STMT ...] " }
.ESAMPLE
.PP
A probe declaration may list multiple comma-separated probe points in
order to attach a handler to all of the named events.  Normally, the
handler statements are run whenever any of events occur.  Depending on
the type of probe point, the handler statements may refer to context
variables (denoted with a dollar-sign prefix like $foo) to read or
write state.  This may include function parameters for function
probes, or local variables for statement probes.
.PP
The syntax of a single probe point is a general dotted-symbol
sequence.  This allows a breakdown of the event namespace into parts,
somewhat like the Domain Name System does on the Internet.  Each
component identifier may be parametrized by a string or number
literal, with a syntax like a function call.  A component may include
a "*" character, to expand to a set of matching probe points.  It may
also include "**" to match multiple sequential components at once.
Probe aliases likewise expand to other probe points.
.PP
Probe aliases can be given on their own, or with a suffix. The suffix
attaches to the underlying probe point that the alias is expanded
to. For example,
.SAMPLE
syscall.read.return.maxactive(10)
.ESAMPLE
expands to
.SAMPLE
kernel.function("sys_read").return.maxactive(10)
.ESAMPLE
with the component
.IR maxactive(10)
being recognized as a suffix.
.PP
Normally, each and every probe point resulting from wildcard- and
alias-expansion must be resolved to some low-level system
instrumentation facility (e.g., a kprobe address, marker, or a timer
configuration), otherwise the elaboration phase will fail.
.PP
However, a probe point may be followed by a "?" character, to indicate
that it is optional, and that no error should result if it fails to
resolve.  Optionalness passes down through all levels of
alias/wildcard expansion.  Alternately, a probe point may be followed
by a "!" character, to indicate that it is both optional and
sufficient.  (Think vaguely of the Prolog cut operator.) If it does
resolve, then no further probe points in the same comma-separated list
will be resolved.  Therefore, the "!"  sufficiency mark only makes
sense in a list of probe point alternatives.
.PP
Additionally, a probe point may be followed by a "if (expr)" statement, in
order to enable/disable the probe point on-the-fly. With the "if" statement,
if the "expr" is false when the probe point is hit, the whole probe body
including alias's body is skipped. The condition is stacked up through
all levels of alias/wildcard expansion. So the final condition becomes
the logical-and of conditions of all expanded alias/wildcard.  The expressions
are necessarily restricted to global variables.
.PP
These are all
.B syntactically
valid probe points.  (They are generally
.B semantically
invalid, depending on the contents of the tapsets, and the versions of
kernel/user software installed.)

.SAMPLE
kernel.function("foo").return
process("/bin/vi").statement(0x2222)
end
syscall.*
syscall.*.return.maxactive(10)
syscall.{open,close}
sys**open
kernel.function("no_such_function") ?
module("awol").function("no_such_function") !
signal.*? if (switch)
kprobe.function("foo")
.ESAMPLE

Probes may be broadly classified into "synchronous" and
"asynchronous".  A "synchronous" event is deemed to occur when any
processor executes an instruction matched by the specification.  This
gives these probes a reference point (instruction address) from which
more contextual data may be available.  Other families of probe points
refer to "asynchronous" events such as timers/counters rolling over,
where there is no fixed reference point that is related.  Each probe
point specification may match multiple locations (for example, using
wildcards or aliases), and all them are then probed.  A probe
declaration may also contain several comma-separated specifications,
all of which are probed.

Brace expansion is a mechanism which allows a list of probe points to be
generated. It is very similar to shell expansion. A component may be surrounded
by a pair of curly braces to indicate that the comma-separated sequence of
one or more subcomponents will each constitute a new probe point. The braces
may be arbitrarily nested. The ordering of expanded results is based on
product order.

The question mark (?), exclamation mark (!) indicators and probe point conditions
may not be placed in any expansions that are before the last component.

The following is an example of brace expansion.

.SAMPLE
syscall.{write,read}
# Expands to
syscall.write, syscall.read

{kernel,module("nfs")}.function("nfs*")!
# Expands to
kernel.function("nfs*")!, module("nfs").function("nfs*")!
.ESAMPLE

.SH DWARF DEBUGINFO

Resolving some probe points requires DWARF debuginfo or "debug
symbols" for the \fIspecific program\fR being instrumented.  For some others,
DWARF is automatically synthesized on the fly from source code header
files.  For others, it is not needed at all.  Since a systemtap script
may use any mixture of probe points together, the union of their DWARF
requirements has to be met on the computer where script compilation
occurs.  (See the \fI\-\-use\-server\fR option and the \fBstap-server\
(8)\fR man page for information about the remote compilation facility,
which allows these requirements to be met on a different machine.)
.PP
The following point lists many of the available probe point families,
to classify them with respect to their need for DWARF debuginfo for
the specific program for that probe point.

.TS
l l l.
\fBDWARF	NON-DWARF	SYMBOL-TABLE\fP

kernel.function, .statement	kernel.mark	kernel.function\fI*\fP
module.function, .statement	process.mark, process.plt	module.function\fI*\fP
process.function, .statement	begin, end, error, never	process.function\fI*\fP
process.mark\fI*\fP	timer
\.function.callee	perf
python2, python3	procfs
debuginfod	kernel.statement.absolute
	kernel.data
\fBAUTO-GENERATED-DWARF\fP	kprobe.function
kernel.trace	process.statement.absolute
	process.begin, .end
	netfilter
	java
.TE

.PP
The probe types marked with \fI*\fP asterisks mark fallbacks, where
systemtap can sometimes infer subset or substitute information.  In
general, the more symbolic / debugging information available, the
higher quality probing will be available.


.SH ON-THE-FLY ARMING

The following types of probe points may be armed/disarmed on-the-fly
to save overheads during uninteresting times.  Arming conditions may
also be added to other types of probes, but will be treated as a
wrapping conditional and won't benefit from overhead savings.

.TS
l l.
\fBDISARMABLE	exceptions\fP
kernel.function, kernel.statement
module.function, module.statement
process.*.function, process.*.statement
process.*.plt, process.*.mark
timer.	timer.profile
java
.TE 

.SH PROBE POINT FAMILIES

.SS BEGIN/END/ERROR

The probe points
.IR begin " and " end
are defined by the translator to refer to the time of session startup
and shutdown.  All "begin" probe handlers are run, in some sequence,
during the startup of the session.  All global variables will have
been initialized prior to this point.  All "end" probes are run, in
some sequence, during the
.I normal
shutdown of a session, such as in the aftermath of an
.I exit ()
function call, or an interruption from the user.  In the case of an
error-triggered shutdown, "end" probes are not run.  There are no
target variables available in either context.
.PP
If the order of execution among "begin" or "end" probes is significant,
then an optional sequence number may be provided:

.SAMPLE
begin(N)
end(N)
.ESAMPLE

The number N may be positive or negative.  The probe handlers are run in
increasing order, and the order between handlers with the same sequence
number is unspecified.  When "begin" or "end" are given without a
sequence, they are effectively sequence zero.

The
.IR error
probe point is similar to the 
.IR end
probe, except that each such probe handler run when the session ends
after errors have occurred.  In such cases, "end" probes are skipped,
but each "error" probe is still attempted.  This kind of probe can be
used to clean up or emit a "final gasp".  It may also be numerically
parametrized to set a sequence.

.SS NEVER
The probe point
.IR never
is specially defined by the translator to mean "never".  Its probe
handler is never run, though its statements are analyzed for symbol /
type correctness as usual.  This probe point may be useful in
conjunction with optional probes.

.SS SYSCALL and ND_SYSCALL

The
.IR syscall.* " and " nd_syscall.*
aliases define several hundred probes, too many to
detail here.  They are of the general form:

.SAMPLE
syscall.NAME
.br
nd_syscall.NAME
.br
syscall.NAME.return
.br
nd_syscall.NAME.return
.ESAMPLE

Generally, a pair of probes are defined for each normal system call as listed in the
.IR syscalls(2)
manual page, one for entry and one for return.  Those system calls that never
return do not have a corresponding
.IR .return
probe.  The nd_* family of probes are about the same, except it uses 
.B non-DWARF
based searching mechanisms, which may result in a lower quality of symbolic
context data (parameters), and may miss some system calls.  You may want to
try them first, in case kernel debugging information is not immediately available.
.PP
Each probe alias provides a variety of variables. Looking at the tapset source
code is the most reliable way.  Generally, each variable listed in the standard
manual page is made available as a script-level variable, so
.IR syscall.open
exposes
.IR filename ", " flags ", and " mode .
In addition, a standard suite of variables is available at most aliases:
.TP
.IR argstr
A pretty-printed form of the entire argument list, without parentheses.
.TP
.IR name
The name of the system call.
.TP
.IR retval
For return probes, the raw numeric system-call result.
.TP
.IR retstr
For return probes, a pretty-printed string form of the system-call result.
.PP
As usual for probe aliases, these variables are all initialized once
from the underlying $context variables, so that later changes to
$context variables are not automatically reflected.  Not all probe
aliases obey all of these general guidelines.  Please report any
bothersome ones you encounter as a bug.  Note that on some
kernel/userspace architecture combinations (e.g., 32-bit userspace on
64-bit kernel), the underlying $context variables may need explicit
sign extension / masking.  When this is an issue, consider using the
tapset-provided variables instead of raw $context variables.
.PP
If debuginfo availability is a problem, you may try using the
non-DWARF syscall probe aliases instead.  Use the
.IR nd_syscall.
prefix instead of
.IR syscall.
The same context variables are available, as far as possible.
.PP
.IR nd_syscall
probes on kernels that use syscall wrappers to pass arguments via pt_regs
(currently 4.17+ on x86_64 and 4.19+ on aarch64) support syscall argument
writing when guru mode is enabled. If a probe syscall parameter is modified
in the probe body then immediately before the probe exits the parameter's
current value will be written to pt_regs. This overwrites the previous value.
.IR nd_syscall
probes also include two parameters for each of the syscall's string parameters.
One holds a quoted version of the string passed to the syscall. The other holds
an unquoted version of the string intended to be used when modifying the parameter.
If the probe modifies the unquoted string variable then as the probe is about to
exit the contents of this variable will be written to the user space buffer passed
to the syscall. It is the user's responsibility to ensure that this buffer is large
enough to hold the modified string and that it is located in a writable memory
segment.

.SS TIMERS

There are two main types of timer probes: "jiffies" timer probes and
time interval timer probes.

Intervals defined by the standard kernel "jiffies" timer may be used
to trigger probe handlers asynchronously.  Two probe point variants
are supported by the translator:

.SAMPLE
timer.jiffies(N)
timer.jiffies(N).randomize(M)
.ESAMPLE

The probe handler is run every N jiffies (a kernel-defined unit of
time, typically between 1 and 60 ms).  If the "randomize" component is
given, a linearly distributed random value in the range [\-M..+M] is
added to N every time the handler is run.  N is restricted to a
reasonable range (1 to around a million), and M is restricted to be
smaller than N.  There are no target variables provided in either
context.  It is possible for such probes to be run concurrently on
a multi-processor computer.
.PP
Alternatively, intervals may be specified in units of time.
There are two probe point variants similar to the jiffies timer:

.SAMPLE
timer.ms(N)
timer.ms(N).randomize(M)
.ESAMPLE

Here, N and M are specified in milliseconds, but the full options for units
are seconds (s/sec), milliseconds (ms/msec), microseconds (us/usec),
nanoseconds (ns/nsec), and hertz (hz).  Randomization is not supported for
hertz timers.

The actual resolution of the timers depends on the target kernel.  For
kernels prior to 2.6.17, timers are limited to jiffies resolution, so
intervals are rounded up to the nearest jiffies interval.  After 2.6.17,
the implementation uses hrtimers for tighter precision, though the actual
resolution will be arch-dependent.  In either case, if the "randomize"
component is given, then the random value will be added to the interval
before any rounding occurs.
.PP
Profiling timers are also available to provide probes that execute on
all CPUs at the rate of the system tick (CONFIG_HZ) or at a given
frequency (hz). On some kernels, this is a one-concurrent-user-only or
disabled facility, resulting in error \-16 (EBUSY) during probe
registration.

.SAMPLE
timer.profile.tick
timer.profile.freq.hz(N)
.ESAMPLE

Full context information of the interrupted process is available, making
this probe suitable for a time-based sampling profiler.
.PP
It is recommended to use the tapset probe
.IR timer.profile
rather than timer.profile.tick. This probe point behaves identically
to timer.profile.tick when the underlying functionality is available,
and falls back to using perf.sw.cpu_clock on some recent kernels which
lack the corresponding profile timer facility.
.PP
Profiling timers with specified frequencies are only accurate up to around
100 hz. You may need to provide a larger value to achieve the desired
rate.
.PP
Note that if a timer probe is set to fire at a very high rate
and if the probe body is complex, succeeding timer probes can get
skipped, since the time for them to run has already passed. Normally
systemtap reports missed probes, but it will not report these skipped
probes.

.SS DWARF

This family of probe points uses symbolic debugging information for
the target kernel/module/program, as may be found in unstripped
executables, or the separate
.I debuginfo
packages.  They allow placement of probes logically into the execution
path of the target program, by specifying a set of points in the
source or object code.  When a matching statement executes on any
processor, the probe handler is run in that context.
.PP
Probe points in the DWARF family can be identified by the target kernel
module (or user process), source file, line number, function name, or
some combination of these.
.PP
Here is a list of DWARF probe points currently supported:
.SAMPLE
kernel.function(PATTERN)
kernel.function(PATTERN).call
kernel.function(PATTERN).callee(PATTERN)
kernel.function(PATTERN).callee(PATTERN).return
kernel.function(PATTERN).callee(PATTERN).call
kernel.function(PATTERN).callees(DEPTH)
kernel.function(PATTERN).return
kernel.function(PATTERN).inline
kernel.function(PATTERN).label(LPATTERN)
module(MPATTERN).function(PATTERN)
module(MPATTERN).function(PATTERN).call
module(MPATTERN).function(PATTERN).callee(PATTERN)
module(MPATTERN).function(PATTERN).callee(PATTERN).return
module(MPATTERN).function(PATTERN).callee(PATTERN).call
module(MPATTERN).function(PATTERN).callees(DEPTH)
module(MPATTERN).function(PATTERN).return
module(MPATTERN).function(PATTERN).inline
module(MPATTERN).function(PATTERN).label(LPATTERN)
kernel.statement(PATTERN)
kernel.statement(PATTERN).nearest
kernel.statement(ADDRESS).absolute
module(MPATTERN).statement(PATTERN)
process("PATH").function("NAME")
process("PATH").statement("*@FILE.c:123")
process("PATH").library("PATH").function("NAME")
process("PATH").library("PATH").statement("*@FILE.c:123")
process("PATH").library("PATH").statement("*@FILE.c:123").nearest
process("PATH").function("*").return
process("PATH").function("myfun").label("foo")
process("PATH").function("foo").callee("bar")
process("PATH").function("foo").callee("bar").return
process("PATH").function("foo").callee("bar").call
process("PATH").function("foo").callees(DEPTH)
process(PID).function("NAME")
process(PID).function("myfun").label("foo")
process(PID).plt("NAME")
process(PID).plt("NAME").return
process(PID).statement("*@FILE.c:123")
process(PID).statement("*@FILE.c:123").nearest
process(PID).statement(ADDRESS).absolute
debuginfod.process("PATH").**
.ESAMPLE
(See the USER-SPACE section below for more information on the process
probes.)
.PP
The list above includes multiple variants and modifiers which provide
additional functionality or filters. They are:
.RS
.TP
\fB.function\fR
Places a probe near the beginning of the named function, so that
parameters are available as context variables.
.TP
\fB.return\fR
Places a probe at the moment \fBafter\fR the return from the named
function, so the return value is available as the "$return" context
variable.
.TP
\fB.inline\fR
Filters the results to include only instances of inlined functions. Note
that inlined functions do not have an identifiable return point, so
\fB.return\fR is not supported on \fB.inline\fR probes.
.TP
\fB.call\fR
Filters the results to include only non-inlined functions (the opposite
set of \fB.inline\fR)
.TP
\fB.exported\fR
Filters the results to include only exported functions.
.TP
\fB.statement\fR
Places a probe at the exact spot, exposing those local variables that
are visible there.
.TP
\fB.statement.nearest\fR
Places a probe at the nearest available line number for each line number
given in the statement.
.TP
\fB.callee\fR
Places a probe on the callee function given in the \fB.callee\fR
modifier, where the callee must be a function called by the target
function given in \fB.function\fR. The advantage of doing this over
directly probing the callee function is that this probe point is run
only when the callee is called from the target function (add the
-DSTAP_CALLEE_MATCHALL directive to override this when calling
\fBstap\fR(1)).

Note that only callees that can be statically determined are available.
For example, calls through function pointers are not available.
Additionally, calls to functions located in other objects (e.g.
libraries) are not available (instead use another probe point). This
feature will only work for code compiled with GCC 4.7+.
.TP
\fB.callees\fR
Shortcut for \fB.callee("*")\fR, which places a probe on all callees of
the function.
.TP
\fB.callees\fR(DEPTH)
Recursively places probes on callees. For example, \fB.callees(2)\fR
will probe both callees of the target function, as well as callees of
those callees. And \fB.callees(3)\fR goes one level deeper, etc...
A callee probe at depth N is only triggered when the N callers in the
callstack match those that were statically determined during analysis
(this also may be overridden using -DSTAP_CALLEE_MATCHALL).
.RE
.PP
In the above list of probe points, MPATTERN stands for a string literal
that aims to identify the loaded kernel module of interest. For in-tree
kernel modules, the name suffices (e.g. "btrfs"). The name may also
include the "*", "[]", and "?" wildcards to match multiple in-tree
modules. Out-of-tree modules are also supported by specifying the full
path to the ko file. Wildcards are not supported. The file must follow
the convention of being named <module_name>.ko (characters ',' and '-'
are replaced by '_').
.PP
LPATTERN stands for a source program label. It may also contain "*",
"[]", and "?" wildcards. PATTERN stands for a string literal that aims
to identify a point in the program.  It is made up of three parts:
.IP \(bu 4
The first part is the name of a function, as would appear in the
.I nm
program's output.  This part may use the "*" and "?" wildcarding
operators to match multiple names.
.IP \(bu 4
The second part is optional and begins with the "@" character. 
It is followed by the path to the source file containing the function,
which may include a wildcard pattern, such as mm/slab*.
If it does not match as is, an implicit "*/" is optionally added
.I before
the pattern, so that a script need only name the last few components
of a possibly long source directory path.
.IP \(bu 4
Finally, the third part is optional if the file name part was given,
and identifies the line number in the source file preceded by a ":"
or a "+".  The line number is assumed to be an
absolute line number if preceded by a ":", or relative to the
declaration line of the function if preceded by a "+".
All the lines in the function can be matched with ":*".  
A range of lines x through y can be matched with ":x\-y". Ranges and
specific lines can be mixed using commas, e.g. ":x,y\-z".
.PP
As an alternative, PATTERN may be a numeric constant, indicating an
address.  Such an address may be found from symbol tables of the
appropriate kernel / module object file.  It is verified against
known statement code boundaries, and will be relocated for use at
run time.
.PP
In guru mode only, absolute kernel-space addresses may be specified with
the ".absolute" suffix.  Such an address is considered already relocated,
as if it came from 
.BR /proc/kallsyms ,
so it cannot be checked against statement/instruction boundaries.

.SS CONTEXT VARIABLES

.PP
Many of the source-level context variables, such as function parameters,
locals, globals visible in the compilation unit, may be visible to
probe handlers.  They may refer to these variables by prefixing their
name with "$" within the scripts.  In addition, a special syntax
allows limited traversal of structures, pointers, and arrays.  More
syntax allows pretty-printing of individual variables or their groups.
See also 
.BR @cast .
Note that variables may be inaccessible due to them being paged out,
or for a few other reasons.  See also man
.IR error::fault (7stap).

.PP
Functions called from DWARF class probe points and from process.mark
probes may also refer to context variables.

.TP
$var
refers to an in-scope variable or thread local storage variable 
"var".  If it's an integer-like type, 
it will be cast to a 64-bit int for systemtap script use.  String-like
pointers (char *) may be copied to systemtap string values using the
.IR kernel_string " or " user_string
functions.
.TP
@var("varname")
an alternative syntax for
.IR $varname
.
.TP
@var("varname","module")
The global variable or global thread local storage variable 
in scope of the given module already loaded into
the current probed process.  Useful to get an exported variable in a
shared library loaded into the process being probed, or a global
variable in a process while a shared library probe is being executed.
For user-space modules only.  For example:
.IR @var("_r_debug","/lib/ld-linux.so.2")
.
.TP
@var("varname@src/file.c")
refers to the global (either file local or external) variable
.IR varname
defined when the file
.IR src/file.c
was compiled. The CU in which the variable is resolved is the first CU
in the module of the probe point which matches the given file name at
the end and has the shortest file name path (e.g. given
.IR @var("foo@bar/baz.c")
and CUs with file name paths
.IR src/sub/module/bar/baz.c
and
.IR src/bar/baz.c
the second CU will be chosen to resolve the (file) global variable
.IR foo
.

.TP
@var("varname@src/file.c","module")
The global variable in scope of the given CU, defined in the given module,
even if the variable is static (so the name is not unique without the CU name).

.TP
$var\->field traversal via a structure's or a pointer's field.  This
generalized indirection operator may be repeated to follow more
levels.  Note that the
.IR .
operator is not used for plain structure
members, only 
.IR \->
for both purposes.  (This is because "." is reserved for string
concatenation.) Also note that for direct dereferencing of $var
pointer {kernel,user}_{char,int,...}($var) should be used. (Refer to
stapfuncs(5) for more details.)
.TP
$return
is available in return probes only for functions that are declared
with a return value, which can be determined using @defined($return).
.TP
$var[N]
indexes into an array.  The index given with a literal number or even
an arbitrary numeric expression.
.PP
A number of operators exist for such basic context variable expressions:
.TP
$$vars
expands to a character string that is equivalent to
.SAMPLE
sprintf("parm1=%x ... parmN=%x var1=%x ... varN=%x",
        parm1, ..., parmN, var1, ..., varN)
.ESAMPLE
for each variable in scope at the probe point.  Some values may be
printed as
.IR =?
if their run-time location cannot be found.
.TP
$$locals
expands to a subset of $$vars for only local variables.
.TP
$$parms
expands to a subset of $$vars for only function parameters.
.TP
$$return
is available in return probes only.  It expands to a string that
is equivalent to sprintf("return=%x", $return)
if the probed function has a return value, or else an empty string.
.TP
& $EXPR
expands to the address of the given context variable expression, if it
is addressable.
.TP
@defined($EXPR)
expands to 1 or 0 iff the given context variable expression is resolvable,
for use in conditionals such as   
.SAMPLE
@defined($foo\->bar) ? $foo\->bar : 0
.ESAMPLE
.TP
@probewrite($VAR)
see the PROBES section of \fIstap\fR(1).
.TP
$EXPR$
expands to a string with all of $EXPR's members, equivalent to
.SAMPLE
sprintf("{.a=%i, .b=%u, .c={...}, .d=[...]}",
         $EXPR\->a, $EXPR\->b)
.ESAMPLE
.TP
$EXPR$$
expands to a string with all of $var's members and submembers, equivalent to
.SAMPLE
sprintf("{.a=%i, .b=%u, .c={.x=%p, .y=%c}, .d=[%i, ...]}",
        $EXPR\->a, $EXPR\->b, $EXPR\->c\->x, $EXPR\->c\->y, $EXPR\->d[0])
.ESAMPLE
.TP
@errno
expands to the last value the C library global variable errno was set to.

.SS MORE ON RETURN PROBES

.PP
For the kernel ".return" probes, only a certain fixed number of
returns may be outstanding.  The default is a relatively small number,
on the order of a few times the number of physical CPUs.  If many
different threads concurrently call the same blocking function, such
as futex(2) or read(2), this limit could be exceeded, and skipped
"kretprobes" would be reported by "stap \-t".  To work around this,
specify a
.SAMPLE
probe FOO.return.maxactive(NNN)
.ESAMPLE
suffix, with a large enough NNN to cover all expected concurrently blocked
threads.  Alternately, use the
.SAMPLE
stap \-DKRETACTIVE=NNNN
.ESAMPLE
stap command line macro setting to override the default for all
".return" probes.

.PP
For ".return" probes, context variables other than the "$return" may
be accessible, as a convenience for a script programmer wishing to
access function parameters.  These values are \fBsnapshots\fP
taken at the time of function entry.  (Local variables within the
function are \fBnot\fP generally accessible, since those variables did
not exist in allocated/initialized form at the snapshot moment.)
These entry-snapshot variables should be accessed via
.IR @entry($var) .
.PP
In addition, arbitrary entry-time expressions can also be saved for
".return" probes using the
.IR @entry(expr)
operator.  For example, one can compute the elapsed time of a function:
.SAMPLE
probe kernel.function("do_filp_open").return {
    println( get_timeofday_us() \- @entry(get_timeofday_us()) )
}
.ESAMPLE

.PP
The following table summarizes how values related to a function
parameter context variable, a pointer named \fBaddr\fP, may be
accessed from a
.IR .return
probe.
.\" summarized from http://sourceware.org/ml/systemtap/2012-q1/msg00025.html
.TS
l l l.
\fBat-entry value	past-exit value\fP

$addr	\fInot available\fP
$addr->x->y	@cast(@entry($addr),"struct zz")->x->y
$addr[0]	{kernel,user}_{char,int,...}(& $addr[0])
.TE


.SS DWARFLESS
In absence of debugging information, entry & exit points of kernel & module
functions can be probed using the "kprobe" family of probes.
However, these do not permit looking up the arguments / local variables
of the function.
Following constructs are supported :
.SAMPLE
kprobe.function(FUNCTION)
kprobe.function(FUNCTION).call
kprobe.function(FUNCTION).return
kprobe.module(NAME).function(FUNCTION)
kprobe.module(NAME).function(FUNCTION).call
kprobe.module(NAME).function(FUNCTION).return
kprobe.statement(ADDRESS).absolute
.ESAMPLE
.PP
Probes of type
.B function
are recommended for kernel functions, whereas probes of type
.B module
are recommended for probing functions of the specified module.
In case the absolute address of a kernel or module function is known,
.B statement
probes can be utilized.
.PP
Note that
.I FUNCTION
and
.I MODULE
names
.B must not
contain wildcards, or the probe will not be registered.
Also, statement probes must be run under guru-mode only.


.SS USER-SPACE
Support for user-space probing is available for kernels that are
configured with the utrace extensions, or have the uprobes facility in
linux 3.5.  (Various kernel build configuration options need to be
enabled; systemtap will advise if these are missing.)

.PP
There are several forms.  First, a non-symbolic probe point:
.SAMPLE
process(PID).statement(ADDRESS).absolute
.ESAMPLE
is analogous to 
.IR
kernel.statement(ADDRESS).absolute
in that both use raw (unverified) virtual addresses and provide
no $variables.  The target PID parameter must identify a running
process, and ADDRESS should identify a valid instruction address.
All threads of that process will be probed.
.PP
Second, non-symbolic user-kernel interface events handled by
utrace may be probed:
.SAMPLE
process(PID).begin
process("FULLPATH").begin
process.begin
process(PID).thread.begin
process("FULLPATH").thread.begin
process.thread.begin
process(PID).end
process("FULLPATH").end
process.end
process(PID).thread.end
process("FULLPATH").thread.end
process.thread.end
process(PID).syscall
process("FULLPATH").syscall
process.syscall
process(PID).syscall.return
process("FULLPATH").syscall.return
process.syscall.return
.ESAMPLE

.PP
A
.B process.begin
probe gets called when new process described by PID or FULLPATH gets created.
In addition, it is called once from the context of each preexisting process,
at systemtap script startup.  This is useful to track live processes.
A
.B process.thread.begin
probe gets called when a new thread described by PID or FULLPATH gets created.
A
.B process.end
probe gets called when process described by PID or FULLPATH dies.
A
.B process.thread.end
probe gets called when a thread described by PID or FULLPATH dies.
A
.B process.syscall
probe gets called when a thread described by PID or FULLPATH makes a
system call.  The system call number is available in the
.BR $syscall
context variable, and the first 6 arguments of the system call
are available in the
.BR $argN
(ex. $arg1, $arg2, ...) context variable.
A
.B process.syscall.return
probe gets called when a thread described by PID or FULLPATH returns from a
system call.  The system call number is available in the
.BR $syscall
context variable, and the return value of the system call is available
in the
.BR $return
context variable.
A

.PP
If a process probe is specified without a PID or FULLPATH, all user
threads will be probed.  However, if systemtap was invoked with the
.IR \-c " or " \-x
options, then process probes are restricted to the process
hierarchy associated with the target process.  If a process probe is
unspecified (i.e. without a PID or FULLPATH), but with the
.IR \-c "
option, the PATH of the
.IR \-c "
cmd will be heuristically filled into the process PATH. In that case,
only command parameters are allowed in the \fI-c\fR command (i.e. no
command substitution allowed and no occurrences of any of these
characters: '|&;<>(){}').

.PP
Third, symbolic static instrumentation compiled into programs and
shared libraries may be
probed:
.SAMPLE
process("PATH").mark("LABEL")
process("PATH").provider("PROVIDER").mark("LABEL")
process(PID).mark("LABEL")
process(PID).provider("PROVIDER").mark("LABEL")
.ESAMPLE
.PP
A
.B .mark
probe gets called via a static probe which is defined in the
application by STAP_PROBE1(PROVIDER,LABEL,arg1), which are macros defined in
.BR sys/sdt.h .
The PROVIDER is an arbitrary application identifier, LABEL is the
marker site identifier, and arg1 is the integer-typed argument.
STAP_PROBE1 is used for probes with 1 argument, STAP_PROBE2 is used
for probes with 2 arguments, and so on.  The arguments of the probe
are available in the context variables $arg1, $arg2, ...  An
alternative to using the STAP_PROBE macros is to use the dtrace script
to create custom macros.  Additionally, the variables $$name and
$$provider are available as parts of the probe point name.  The 
.B sys/sdt.h 
macro names DTRACE_PROBE* are available as aliases for STAP_PROBE*.

.PP
Finally, full symbolic source-level probes in user-space programs and
shared libraries are supported.  These are exactly analogous to the
symbolic DWARF-based kernel/module probes described above.  They
expose the same sorts of context $variables for function parameters,
local variables, and so on.
.SAMPLE
process("PATH").function("NAME")
process("PATH").statement("*@FILE.c:123")
process("PATH").plt("NAME")
process("PATH").library("PATH").plt("NAME")
process("PATH").library("PATH").function("NAME")
process("PATH").library("PATH").statement("*@FILE.c:123")
process("PATH").function("*").return
process("PATH").function("myfun").label("foo")
process("PATH").function("foo").callee("bar")
process("PATH").plt("NAME").return
debuginfod.process("PATH").**
process(PID).function("NAME")
process(PID).statement("*@FILE.c:123")
process(PID).plt("NAME")
.ESAMPLE

.PP
Note that for all process probes,
.I PATH
names refer to executables that are searched the same way shells do: relative
to the working directory if they contain a "/" character, otherwise in 
.BR $PATH .
If PATH names refer to scripts, the actual interpreters (specified in the
script in the first line after the #! characters) are probed.
In the debuginfod probe family
.I PATH
names likewise refer to executables, but are searched for in the currently
defined
.BR $DEBUGINFOD_URLS .


.PP
Tapset process probes placed in the special directory
$prefix/share/systemtap/tapset/PATH/ with relative paths will have their
process parameter prefixed with the location of the tapset. For example,

.SAMPLE
process("foo").function("NAME")
.ESAMPLE
.PP
expands to
.SAMPLE
process("/usr/bin/foo").function("NAME")
.ESAMPLE

.PP
when placed in $prefix/share/systemtap/tapset/PATH/usr/bin/

.PP
If PATH is a process component parameter referring to shared libraries
then all processes that map it at runtime would be selected for probing.
If PATH is a library component parameter referring to shared libraries
then the process specified by the process component would be selected.
Note that the PATH pattern in a library component will always apply to
libraries statically determined to be in use by the process. However,
you may also specify the full path to any library file even if not
statically needed by the process.

.PP
A .plt probe will probe functions in the program linkage table
corresponding to the rest of the probe point.  .plt can be specified
as a shorthand for .plt("*").  The symbol name is available as a
$$name context variable; function arguments are not available, since
PLTs are processed without debuginfo.  A .plt.return probe places a
probe at the moment \fBafter\fR the return from the named 
function.

.PP
If the PATH string contains wildcards as in the MPATTERN case, then
standard globbing is performed to find all matching paths.  In this
case, the 
.BR $PATH
environment variable is not used.

.PP
If systemtap was invoked with the
.IR \-c " or " \-x
options, then process probes are restricted to the process
hierarchy associated with the target process.

.SS DEBUGINFOD
These probes take the form
.SAMPLE
debuginfod.process("PATH").**
.ESAMPLE
.PP
They are very similar to the process("PATH").** probe family.
The key difference is that the process probes search for
.I PATH
in the host filesystem, while debuginfod probes
search the current federation of debuginfod servers, using the
currently defined
.BR $DEBUGINFOD_URLS 
(see
.I debuginfod(8)
).

.PP
In order to probe the contents of one or more elf/archive files
and/or elf/archive containing directories, the below will create
a debuginfod server which will scan and process the elf files within
and prepare them for systemtap.
.SAMPLE
$ debuginfod [options] [-F -R -Z etc.] /path1 /path2
$ env DEBUGINFOD_URLS=http://localhost:8002/ stap ...
.ESAMPLE

.SS JAVA
Support for probing Java methods is available using Byteman as a
backend. Byteman is an instrumentation tool from the JBoss project
which systemtap can use to monitor invocations for a specific method
or line in a Java program.
.PP
Systemtap does so by generating a Byteman script listing the probes to
instrument and then invoking the Byteman
.IR bminstall
utility.
.PP
This Java instrumentation support is currently a prototype feature
with major limitations.  Moreover, Java probing currently does not
work across users; the stap script must run (with appropriate
permissions) under the same user that the Java process being
probed. (Thus a stap script under root currently cannot probe Java
methods in a non-root-user Java process.)

.PP
The first probe type refers to Java processes by the name of the Java process:
.SAMPLE
java("PNAME").class("CLASSNAME").method("PATTERN")
java("PNAME").class("CLASSNAME").method("PATTERN").return
.ESAMPLE
The PNAME argument must be a pre-existing jvm pid, and be identifiable
via a jps listing.
.PP
The PATTERN parameter specifies the signature of the Java method to
probe. The signature must consist of the exact name of the method,
followed by a bracketed list of the types of the arguments, for
instance "myMethod(int,double,Foo)". Wildcards are not supported.
.PP
The probe can be set to trigger at a specific line within the method
by appending a line number with colon, just as in other types of
probes: "myMethod(int,double,Foo):245".
.PP
The CLASSNAME parameter identifies the Java class the method belongs
to, either with or without the package qualification. By default, the
probe only triggers on descendants of the class that do not override
the method definition of the original class. However, CLASSNAME can
take an optional caret prefix, as in
.IR ^org.my.MyClass,
which specifies that the probe should also trigger on all descendants
of MyClass that override the original method. For instance, every method
with signature foo(int) in program org.my.MyApp can be probed at once using
.SAMPLE
java("org.my.MyApp").class("^java.lang.Object").method("foo(int)")
.ESAMPLE
.PP
The second probe type works analogously, but refers to Java processes by PID:
.SAMPLE
java(PID).class("CLASSNAME").method("PATTERN")
java(PID).class("CLASSNAME").method("PATTERN").return
.ESAMPLE
(PIDs for an already running process can be obtained using the
.IR jps (1)
utility.)
.PP
Context variables defined within java probes include
.IR $arg1
through
.IR $arg10
(for up to the first 10 arguments of a method), represented as character-pointers
for the
.B toString()
form of each actual argument.
The
.IR arg1
through
.IR arg10
script variables provide access to these as ordinary strings, fetched via
.IR user_string_warn() .
.PP
Prior to systemtap version 3.1,
.IR $arg1
through
.IR $arg10
could contain either integers or character pointers, depending on the types of the
objects being passed to each particular java method.  This previous behaviour may
be invoked with the
.I stap --compatible=3.0
flag.

.SS PROCFS

These probe points allow procfs "files" in
/proc/systemtap/MODNAME to be created, read and written using a  
permission that may be modified using the proper umask value. Default permissions are 0400 for read
probes, and 0200 for write probes. If both a read and write probe are being 
used on the same file, a default permission of 0600 will be used.
Using procfs.umask(0040).read would
result in a 0404 permission set for the file.
.RI ( MODNAME
is the name of the systemtap module). The
.I proc
filesystem is a pseudo-filesystem which is used as an interface to
kernel data structures. There are several probe point variants supported
by the translator:

.SAMPLE
procfs("PATH").read
procfs("PATH").umask(UMASK).read
procfs("PATH").read.maxsize(MAXSIZE)
procfs("PATH").umask(UMASK).maxsize(MAXSIZE)
procfs("PATH").write
procfs("PATH").umask(UMASK).write
procfs.read
procfs.umask(UMASK).read
procfs.read.maxsize(MAXSIZE)
procfs.umask(UMASK).read.maxsize(MAXSIZE)
procfs.write
procfs.umask(UMASK).write
.ESAMPLE

Note that there are a few differences when procfs probes are used in the stapbpf runtime. 
.RI FIFO
special files are used instead of proc filesystem files.
These files are created in 
/var/tmp/systemtap-USER/MODNAME.
.RI (USER 
is the name of the user). 
Additionally, users cannot create both read and write probes on the same file.

.I PATH
is the file name (relative to /proc/systemtap/MODNAME or /var/tmp/systemtap-USER/MODNAME) to be created.
If no
.I PATH
is specified (as in the last two variants above),
.I PATH
defaults to "command". The file name "__stdin" is used internally by systemtap
for input probes and should not be used as a
.I PATH
for procfs probes; see the input probe section below.
.PP
When a user reads /proc/systemtap/MODNAME/PATH (normal runtime) or /var/tmp/systemtap-USER/MODNAME (stapbpf runtime), the corresponding
procfs
.I read
probe is triggered.  The string data to be read should be assigned to
a variable named
.IR $value ,
like this:

.SAMPLE
procfs("PATH").read { $value = "100\\n" }
.ESAMPLE
.PP
When a user writes into /proc/systemtap/MODNAME/PATH (normal runtime) or /var/tmp/systemtap-USER/MODNAME (stapbpf runtime), the
corresponding procfs
.I write
probe is triggered.  The data the user wrote is available in the
string variable named
.IR $value ,
like this:

.SAMPLE
procfs("PATH").write { printf("user wrote: %s", $value) }
.ESAMPLE
.PP
.I MAXSIZE
is the size of the procfs read buffer.  Specifying
.I MAXSIZE
allows larger procfs output.  If no
.I MAXSIZE
is specified, the procfs read buffer defaults to
.I STP_PROCFS_BUFSIZE
(which defaults to
.IR MAXSTRINGLEN ,
the maximum length of a string).
If setting the procfs read buffers for more than one file is needed,
it may be easiest to override the
.I STP_PROCFS_BUFSIZE
definition.
Here's an example of using
.IR MAXSIZE :

.SAMPLE
procfs.read.maxsize(1024) {
    $value = "long string..."
    $value .= "another long string..."
    $value .= "another long string..."
    $value .= "another long string..."
}
.ESAMPLE

.SS INPUT

These probe points make input from stdin available to the script during runtime.
The translator currently supports two variants of this family:
.SAMPLE
input.char
input.line
.ESAMPLE

.BR input.char
is triggered each time a character is read from stdin. The current
character is available in the string variable named
.IR char .
There is no newline buffering; the next character is read from stdin as soon
as it becomes available.

.BR input.line
causes all characters read from stdin to be buffered until a newline is read,
at which point the probe will be triggered. The current line of characters
(including the newline) is made available in a string variable named
.IR line .
Note that no more than MAXSTRINGLEN characters will be buffered. Any additional
characters will not be included in
.IR line .

.PP
Input probes are aliases for
.BR procfs("__stdin").write .
Systemtap reconfigures stdin if the presence of this procfs probe is detected,
therefore "__stdin" should not be used as a path argument for procfs probes.
Additionally, input probes will not work with the -F and --remote options.

.SS NETFILTER HOOKS

These probe points allow observation of network packets using the
netfilter mechanism. A netfilter probe in systemtap corresponds to a
netfilter hook function in the original netfilter probes API. It is
probably more convenient to use
.IR tapset::netfilter (3stap),
which wraps the primitive netfilter hooks and does the work of
extracting useful information from the context variables.

.PP
There are several probe point variants supported by the translator:

.SAMPLE
netfilter.hook("HOOKNAME").pf("PROTOCOL_F")
netfilter.pf("PROTOCOL_F").hook("HOOKNAME")
netfilter.hook("HOOKNAME").pf("PROTOCOL_F").priority("PRIORITY")
netfilter.pf("PROTOCOL_F").hook("HOOKNAME").priority("PRIORITY")
.ESAMPLE

.PP
.I PROTOCOL_F
is the protocol family to listen for, currently one of
.I NFPROTO_IPV4,
.I NFPROTO_IPV6,
.I NFPROTO_ARP,
or
.I NFPROTO_BRIDGE.

.PP
.I HOOKNAME
is the point, or 'hook', in the protocol stack at which to intercept
the packet. The available hook names for each protocol family are
taken from the kernel header files <linux/netfilter_ipv4.h>,
<linux/netfilter_ipv6.h>, <linux/netfilter_arp.h> and
<linux/netfilter_bridge.h>. For instance, allowable hook names for
.I NFPROTO_IPV4
are
.I NF_INET_PRE_ROUTING,
.I NF_INET_LOCAL_IN,
.I NF_INET_FORWARD,
.I NF_INET_LOCAL_OUT,
and
.I NF_INET_POST_ROUTING.

.PP
.I PRIORITY
is an integer priority giving the order in which the probe point
should be triggered relative to any other netfilter hook functions
which trigger on the same packet. Hook functions execute on each
packet in order from smallest priority number to largest priority number. If no
.I PRIORITY
is specified (as in the first two probe point variants above),
.I PRIORITY
defaults to "0".

There are a number of predefined priority names of the form
.I NF_IP_PRI_*
and
.I NF_IP6_PRI_*
which are defined in the kernel header files <linux/netfilter_ipv4.h> and <linux/netfilter_ipv6.h> respectively. The script is permitted to use these
instead of specifying an integer priority. (The probe points for
.I NFPROTO_ARP
and
.I NFPROTO_BRIDGE
currently do not expose any named hook priorities to the script writer.)
Thus, allowable ways to specify the priority include:

.SAMPLE
priority("255")
priority("NF_IP_PRI_SELINUX_LAST")
.ESAMPLE

A script using guru mode is permitted to specify any identifier or
number as the parameter for hook, pf, and priority. This feature
should be used with caution, as the parameter is inserted verbatim into
the C code generated by systemtap.

The netfilter probe points define the following context variables:
.TP
.IR $hooknum
The hook number.
.TP
.IR $skb
The address of the sk_buff struct representing the packet. See
<linux/skbuff.h> for details on how to use this struct, or
alternatively use the tapset
.IR tapset::netfilter (3stap)
for easy access to key information.

.TP
.IR $in
The address of the net_device struct representing the network device
on which the packet was received (if any). May be 0 if the device is
unknown or undefined at that stage in the protocol stack.

.TP
.IR $out
The address of the net_device struct representing the network device
on which the packet will be sent (if any). May be 0 if the device is
unknown or undefined at that stage in the protocol stack. 

.TP
.IR $verdict
(Guru mode only.) Assigning one of the verdict values defined in
<linux/netfilter.h> to this variable alters the further progress of
the packet through the protocol stack. For instance, the following
guru mode script forces all ipv6 network packets to be dropped:

.SAMPLE
probe netfilter.pf("NFPROTO_IPV6").hook("NF_IP6_PRE_ROUTING") {
  $verdict = 0 /* nf_drop */
}
.ESAMPLE

For convenience, unlike the primitive probe points discussed here, the
probes defined in
.IR tapset::netfilter (3stap)
export the lowercase names of the verdict constants (e.g. NF_DROP
becomes nf_drop) as local variables.

.SS KERNEL TRACEPOINTS

This family of probe points hooks up to static probing tracepoints
inserted into the kernel or modules.  As with markers, these
tracepoints are special macro calls inserted by kernel developers to
make probing faster and more reliable than with DWARF-based probes,
and DWARF debugging information is not required to probe tracepoints.
Tracepoints have an extra advantage of more strongly-typed parameters
than markers.

Tracepoint probes look like:
.BR kernel.trace("name") .
The tracepoint name string, which may contain the usual wildcard
characters, is matched against the names defined by the kernel
developers in the tracepoint header files. To restrict the search to
specific subsystems (e.g. sched, ext3, etc...), the following syntax
can be used:
.BR kernel.trace("system:name") .
The tracepoint system string may also contain the usual wildcard
characters.

The handler associated with a tracepoint-based probe may read the
optional parameters specified at the macro call site.  These are
named according to the declaration by the tracepoint author.  For
example, the tracepoint probe
.BR kernel.trace("sched:sched_switch")
provides the parameters
.BR $prev " and " $next .
If the parameter is a complex type, as in a struct pointer, then a
script can access fields with the same syntax as DWARF $target
variables.  Also, tracepoint parameters cannot be modified, but in
guru-mode a script may modify fields of parameters.

The subsystem and name of the tracepoint are available in
.BR $$system " and " $$name
and a string of name=value pairs for all parameters of the tracepoint
is available in
.BR $$vars " or " $$parms .

.SS KERNEL MARKERS (OBSOLETE)

This family of probe points hooks up to an older style of static
probing markers inserted into older kernels or modules.  These markers
are special STAP_MARK macro calls inserted by kernel developers to
make probing faster and more reliable than with DWARF-based probes.
Further, DWARF debugging information is
.I not
required to probe markers.  

Marker probe points begin with 
.BR kernel .
The next part names the marker itself:
.BR mark("name") .
The marker name string, which may contain the usual wildcard characters,
is matched against the names given to the marker macros when the kernel
and/or module was compiled.    Optionally, you can specify
.BR format("format") .
Specifying the marker format string allows differentiation between two
markers with the same name but different marker format strings.

The handler associated with a marker-based probe may read the
optional parameters specified at the macro call site.  These are
named
.BR $arg1 " through " $argNN ,
where NN is the number of parameters supplied by the macro.  Number
and string parameters are passed in a type-safe manner.

The marker format string associated with a marker is available in
.BR $format .
And also the marker name string is available in
.BR $name .

.SS HARDWARE BREAKPOINTS
This family of probes is used to set hardware watchpoints for a given
 (global) kernel symbol. The probes take three components as inputs :

1. The
.BR virtual
address / name of the kernel symbol to be traced is supplied as
argument to this class of probes. ( Probes for only data segment
variables are supported. Probing local variables of a function cannot
be done.)

2. Nature of access to be probed :
a.
.I .write
probe gets triggered when a write happens at the specified address/symbol
name.
b.
.I rw
probe is triggered when either a read or write happens.

3.
.BR .length
(optional)
Users have the option of specifying the address interval to be probed
using "length" constructs. The user-specified length gets approximated
to the closest possible address length that the architecture can
support. If the specified length exceeds the limits imposed by
architecture, an error message is flagged and probe registration fails.
Wherever 'length' is not specified, the translator requests a hardware
breakpoint probe of length 1. It should be noted that the "length"
construct is not valid with symbol names.

Following constructs are supported :
.SAMPLE
probe kernel.data(ADDRESS).write
probe kernel.data(ADDRESS).rw
probe kernel.data(ADDRESS).length(LEN).write
probe kernel.data(ADDRESS).length(LEN).rw
probe kernel.data("SYMBOL_NAME").write
probe kernel.data("SYMBOL_NAME").rw
.ESAMPLE

This set of probes make use of the debug registers of the processor,
which is a scarce resource. (4 on x86 , 1 on powerpc ) The script
translation flags a warning if a user requests more hardware breakpoint probes
than the limits set by architecture. For example,a pass-2 warning is flashed
when an input script requests 5 hardware breakpoint probes on an x86
system while x86 architecture supports a maximum of 4 breakpoints.
Users are cautioned to set probes judiciously.

.SS PERF

This family of probe points interfaces to the kernel "perf event"
infrastructure for controlling hardware performance counters.
The events being attached to are described by the "type",
"config" fields of the 
.IR perf_event_attr
structure, and are sampled at an interval governed by the
"sample_period" and "sample_freq" fields.

These fields are made available to systemtap scripts using
the following syntax:
.SAMPLE
probe perf.type(NN).config(MM).sample(XX)
probe perf.type(NN).config(MM).hz(XX)
probe perf.type(NN).config(MM)
probe perf.type(NN).config(MM).process("PROC")
probe perf.type(NN).config(MM).counter("COUNTER")
probe perf.type(NN).config(MM).process("PROC").counter("NAME")
.ESAMPLE
The systemtap probe handler is called once per XX increments
of the underlying performance counter when using the .sample field
or at a frequency in hertz when using the .hz field. When not specified,
the default behavior is to sample at a count of 1000000.
The range of valid type/config is described by the 
.IR perf_event_open (2)
system call, and/or the 
.IR linux/perf_event.h
file.  Invalid combinations or exhausted hardware counter resources
result in errors during systemtap script startup.  Systemtap does
not sanity-check the values: it merely passes them through to
the kernel for error- and safety-checking.  By default the perf event
probe is systemwide unless .process is specified, which will bind the
probe to a specific task.  If the name is omitted then it
is inferred from the stap \-c argument.   A perf event can be read on
demand using .counter.  The body of the perf probe handler will not be
invoked for a .counter probe; instead, the counter is read in a user
space probe via:
.TP
   process("PROC").statement("func@file") {stat <<< @perf("NAME")} 


.SS PYTHON
Support for probing python 2 and python 3 function is available with
the help of an extra python support module. Note that the debuginfo
for the version of python being probed is required. To run a python
script with the extra python support module you'd add the '-m
HelperSDT' option to your python command, like this:
.SAMPLE
stap foo.stp -c "python -m HelperSDT foo.py"
.ESAMPLE
Python probes look like the following:
.SAMPLE
python2.module("MPATTERN").function("PATTERN")
python2.module("MPATTERN").function("PATTERN").call
python2.module("MPATTERN").function("PATTERN").return
python3.module("MPATTERN").function("PATTERN")
python3.module("MPATTERN").function("PATTERN").call
python3.module("MPATTERN").function("PATTERN").return
.ESAMPLE
The list above includes multiple variants and modifiers which provide
additional functionality or filters. They are:
.RS
.TP
\fB.function\fR
Places a probe at the beginning of the named function by
default, unless modified by PATTERN. Parameters are available as
context variables.
.TP
\fB.call\fR
Places a probe at the beginning of the named function. Parameters are
available as context variables.
.TP
\fB.return\fR
Places a probe at the moment \fBbefore\fR the return from the named
function. Parameters and local/global python variables are available
as context variables.
.RE
.PP
PATTERN stands for a string literal that aims to identify a point in
the python program.  It is made up of three parts:
.IP \(bu 4
The first part is the name of a function (e.g. "foo") or class method
(e.g. "bar.baz"). This part may use the "*" and "?" wildcarding
operators to match multiple names.
.IP \(bu 4
The second part is optional and begins with the "@" character. 
It is followed by the path to the source file containing the function,
which may include a wildcard pattern. The python path is searched for
a matching filename.
.IP \(bu 4
Finally, the third part is optional if the file name part was given,
and identifies the line number in the source file preceded by a ":"
or a "+".  The line number is assumed to be an
absolute line number if preceded by a ":", or relative to the
declaration line of the function if preceded by a "+".
All the lines in the function can be matched with ":*".  
A range of lines x through y can be matched with ":x\-y". Ranges and
specific lines can be mixed using commas, e.g. ":x,y\-z".
.PP
In the above list of probe points, MPATTERN stands for a python module
or script name that names the python module of interest. This part may
use the "*" and "?" wildcarding operators to match multiple names. The
python path is searched for a matching filename.


.SH EXAMPLES
.PP
Here are some example probe points, defining the associated events.
.TP
begin, end, end
refers to the startup and normal shutdown of the session.  In this
case, the handler would run once during startup and twice during
shutdown.
.TP
timer.jiffies(1000).randomize(200)
refers to a periodic interrupt, every 1000 +/\- 200 jiffies.
.TP
kernel.function("*init*"), kernel.function("*exit*")
refers to all kernel functions with "init" or "exit" in the name.
.TP
kernel.function("*@kernel/time.c:240")
refers to any functions within the "kernel/time.c" file that span
line 240.  
.BR
Note
that this is
.BR not
a probe at the statement at that line number.  Use the
.IR
kernel.statement
probe instead.
.TP
kernel.trace("sched_*")
refers to all scheduler-related (really, prefixed) tracepoints in
the kernel.
.TP
kernel.mark("getuid")
refers to an obsolete STAP_MARK(getuid, ...) macro call in the kernel.
.TP
module("usb*").function("*sync*").return
refers to the moment of return from all functions with "sync" in the
name in any of the USB drivers.
.TP
kernel.statement(0xc0044852)
refers to the first byte of the statement whose compiled instructions
include the given address in the kernel.
.TP
kernel.statement("*@kernel/time.c:296")
refers to the statement of line 296 within "kernel/time.c".
.TP
kernel.statement("bio_init@fs/bio.c+3")
refers to the statement at line bio_init+3 within "fs/bio.c".
.TP
kernel.data("pid_max").write
refers to a hardware breakpoint of type "write" set on pid_max
.TP
syscall.*.return
refers to the group of probe aliases with any name in the third position

.SH SEE ALSO
.nh
.nf
.IR stap (1),
.IR probe::* (3stap),
.IR tapset::* (3stap)

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